This invention relates to an externally threaded, fiber-reinforced plastic member and a method of producing the same. More particularly, the present invention is concerned with an externally threaded, fiber-reinforced plastic member comprising a shank and an integral thread on the exterior surface on the shank and having a unique interior structure in which most of the resin-impregnated fiber-filaments existing within the thread and extending substantially in a plane including a cross-section of the threaded member are those extending continuously from and laterally of the shank into the thread, and also is concerned with a method producing the same.
Metals or metal alloys such as iron and stainless steel are widely employed as a material for forming threaded members such as bolts and nuts. Metal bolts and nuts are employed as a fastening means in a wide variety of applications including manufacture of playthings and building of large-scale industrial or commercial constructions. However, the use of metal bolts and nuts inevitably encounters serious problems. One of such problems is corrosion. At present, metal bolts and nuts are still often used in corrosive environments. The use of metal bolts and nuts in chemical plants, water or sewage treating plants and the like often leads to fatal dangers. Further, it is noted that metal bolts and nuts are unsuitable for use in marine constructions such as a boat, an artificial reef construction, etc., since they are caused to incessantly come in contact with the sea water, leading to destruction of the constructions. Another important problem resides in electrical and thermal conductivity of the metal bolts and nuts. There are a number of applications where bolts and nuts are required not to be thermally and electrically conductive. Representative examples of such applications are the manufacture of heavy electrical devices or equipment, and construction of electrically-powered transportation devices such as cars for an electrically-powered train, street car, etc. For such applications, metal bolts and nuts must be coated with an insulating material. However, the currently employed insulated metal bolts and nuts are still unsatisfactory.
With respect to corrosion-resistant metals which can be used as a material for bolts and nuts, it is well known that titanium is a metal which is comparatively resistant to corrosion. Further, due to its small specific gravity, titanium is considered to be useful for forming bolts and nuts. However, titanium is not only expensive but also poor in processability. For this reason, the use of titanium-made bolts and nuts are restricted to special fields, for example the aero-space industry. With respect to non-conductive bolts and nuts, there have recently been proposed bolts and nuts made of ceramics due to the rapid progress of the ceramics industry. However, satisfactory ceramic made bolts and nuts have not yet been produced.
In order to cope with the problem of corrosion, bolts and nuts made of a synthetic resin have been proposed. Such resin-made bolts and nuts, however, are very poor in mechanical strength as compared with those made of a metal, and, hence, cannot be an effective substitute for metal bolts and nuts in the application field where a high mechanical strength is required. Therefore, the application of the resin-made bolts and nuts is restricted to a field where the corrosion resistance is strongly required but the mechanical strength is not a matter of importance.
In the field of manufacture of heavy electrical devices or equipment, as a bolt which is required to have excellent mechanical strength and insulating properties, bolts made of fiber-reinforced plastic (hereinafter often abbreviated as "FRP") have been proposed. For example, there has been proposed a FRP made bolt prepared by impregnating a plurality of rovings of about 2000 to about 10000 Tex with a thermohardening resin, drawing the thus obtained resin-impregnated rovings into a mold, curing the resin impregnated rovings in the mold to obtain a rigid rod, and threading the external surface of the rigid rod. This bolt cannot be advantageously used due to such a drawback that since the thread is formed by cutting the external surface of the rod comprising resin-impregnated rovings arranged only longitudinally of the axis of the rod, the rovings left within the thread are fiber filaments cut into extremely short lengths and have no interconnection with the rovings within the shank and, therefore, the thread is poor in mechanical strength and cannot be resistant to the shearing forces imposed thereon.
In order to increase the mechanical strength of the thread of a bolt or the like to a desired level, there has been proposed externally threaded FRP members produced by impregnating with a thermohardening resin, such as an epoxy resin, a yarn cloth made of reinforcing filaments, subjecting the thus obtained resin impregnated yarn cloth to thermohardening in a mold to prepare a rod, and threading the surface of the rod. For example, when an externally threaded FRP member with a diameter larger than about 20 to 25 mm is intended, such a threaded member may be produced by a method comprising impregnating a yarn cloth with a solution of a thermohardening resin in a solvent therefor, heating the above-obtained resin impregnated cloth to remove the solvent, thereby, to obtain a half-cured, substantially non-tacky sheet (hereinafter referred to as "prepreg"), rolling the thus obtained prepreg, heating and pressing the rolled prepreg in a mold to prepare a rigid rod having a circular cross-section, and threading the surface of the rigid rod. On the other hand, when an externally threaded FRP member with a diameter less than about 20 to 25 mm is intended, the threaded member may be produced as follows. According to the so-called prepreg-press method, a predetermined number of the above-mentioned prepregs 1 are piled up as shown in FIG. 1(A) which will be mentioned later, the piled-up prepregs are softened by heating, and the piled-up prepregs are subjected to molding by means of a split die 2 as shown in FIG. 1(B), which will be mentioned later, to prepare a rod 3. The rod thus prepared is subjected, on its peripheral surface, to threading to obtain an FRP made threaded member. As is apparent from the above, both the above-mentioned threaded members are each composed of a thermohardened resin and a yarn cloth comprising a plurality of first strands of fiber filaments arranged substantially in parallel and extending longitudinally of the axis of the shank and a plurality of second strands of fiber filaments arranged substantially in parallel and substantially in perpendicular relation to said first strands. However, in the former, namely, in the externally thread member made of a rolled prepreg, as viewed in a cross-section perpendiacular to the axis of the shank, only one first strand extend from the center of the shank into the thread in a rolled manner and, therefore, the reinforcing effect of the fiber filaments of the strand cannot be attained at the thread portion. On the other hand, in the latter, namely, in the externally threaded member produced by the so-called prepreg-press method, in substantially only both diametrically opposite portions of the externally threaded member corresponding to the boundary portions of the die halves of the split die (see FIG. 1(B)) there concentratedly exist second strands extending continuously from and laterally of the shank into thread. The fiber reinforcing effect of the fiber filaments of the strands at the thread portion has been improved to some extent as compared to that of the above-mentioned threaded member of the rolled prepreg. However, in producing the above-mentioned threaded member, the preparation of prepregs and the molding of the prepregs require troublesome procedures. Therefore, the threaded member thus produced is very expensive so that the production of it needs 3 to 4 times the cost for customary FRP made threaded member.
On the other hand, there has recently been proposed a bolt which is obtained by threading the surface of a rod of dual structure. The rod of dual structure is prepared by impregnating a plurality of glass fiber rovings of about 8000 Tex with a resin, pultruding said plurality of glass fiber rovings through a heated mold to prepare a core rod, winding on the surface of the core rod a chopped strand mat made of 1 to 2 inch cut glass fibers, and subjecting the resulting chopped strand matwound core rod to pultrusion or press-molding to obtain a rod of dual structure. This kind of bolt, however, has such disadvantages that the process for producing the same requires complicated procedures and that the bolt thus produced tends to easily separate between the chopped strand mat layer and the core rod.